The subject matter disclosed herein relates to a multi-cell electrochromic device. More particularly, the subject matter disclosed herein relates to a solid-state multi-cell electrochromic device that provides a lighter weight that conventional multi-cell electrochromic devices, improved independent cell properties, faster switching times, and more uniform coloring in its colored state.
The field of electrochromics is extensive and has been developing over about the last forty years. In one application, an electrochromic coating is used for controlling the amount of light and heat passing through the window based on a user-controlled electrical potential that is applied across the optical stack of the electrochromic coating. Not only can an electrochromic coating reduce the amount of energy used for room heating and/or air conditioning, an electrochromic coating can also be used for providing privacy. By switching between a clear state having an optical transmission of about 60-80% and a darkened state having an optical transmission of between 0.1-10%, both energy flow into a room through a window and privacy provided by the window can be controlled. The amount of glass used for various types of windows, such as skylights, aircraft windows, residential and commercial building windows, and automobile windows, is on the order of one billion square meters per year. Accordingly, the potential energy saving provided by electrochromic glazing is substantial. See, for example, Solar Energy Materials and Solar Cells, (1994) pp. 307-321.
Over the forty years that electrochromics have been developing, various structures for electrochromic devices have been proposed including, solution-phase electrochromic devices, solid-state electrochromic devices, gasochromic devices, and photochromic devices.
For example, a conventional electrochromic cell generally is structured as follows: a substrate, a transparent conductive layer, a counter electrode, an ion transport layer, an electrochromic layer, and a transparent conductive layer. Conventional cathodic materials, commonly referred to as “electrochromic electrodes,” have included tungsten oxide WO3 (most common), vanadium oxide V2O5, niobium oxide Nb2O3 and iridium oxide IrO2. Anodic materials, commonly referred to as “counter electrodes,” include nickel oxide NiO, tungsten-doped nickel oxide, and iridium oxide IrO2. The ion layer materials exhibit a poor electron conductor, but a good ion conductor. Examples of ion layer materials include SiO2, TiO2, Al2O3, and Ta2O5.
Various types of transparent conducting thin films have been employed for the first and second transparent conducting layers, such as, indium tin oxide ITO, which is the most commonly used material. Other thin metal layers have also been used, such as fluorine-doped tin oxide, antimony-doped tin oxide, and fluorine-doped aluminum oxide. Regardless which thin film is used, conductivities of less than about 20 Ohms/□ are needed in order to produce a uniform voltage between the two conductive layers across the conductive layers. Even lower conductivities than about 20 Ohms/□ are needed for large panes of glass measuring 3-4 feet across.
If a voltage of between 1-4 Volts is applied between the first and second transparent conducting layers, the following reactions take place. At the anode, the following reaction takes place:                Metal Oxide or Polymer or Organic Molecule (Colored)+xM++xeMetal Oxide or Polymer or Organic Molecule (Transparent).        
At the cathode, the following reaction takes place:                Metal Oxide or Polymer or Organic Molecule (Transparent)+xM++xe−Metal Oxide or Polymer or Organic Molecule (Colored).        
in which M is H+, Li+ or Na+, e is an electron, and x is an integer.
Conventional single electrochromic cells often exhibit variations in coloring due to non-uniform fields and differential ion migration. This is especially true for large area glazings in which resistances of the conductive coating layers cause voltage drops between the center of the window pane and the bus bars supplying the voltage.
In an effort to mitigate these difficulties, two approaches have been tried. U.S. Pat. No. 7,256,924 B2 to Guarr et al., which is incorporated herein by reference, discloses two electrochromic cells in optical alignment. Each cell is enclosed by substrates on each side so that for two cells, a total of four substrates are required. Each cell is also enclosed with top and bottom sealants and the cell cavity contains at least one solvent or gel-like material. Electrical power to the multi-cells is placed in series or in parallel with the two cells.
U.S. Pat. No. 5,076,673 to Lynam et al. also discloses a multi-cell electrochromic structure. This structure, like the structures disclosed in U.S. Pat. No. 7,256,924 B2 to Guarr et al., also has each individual cell encased with two substrates (in this case, glass) and top and bottom sealants. The cells are configured to alternatively color and bleach with an applied electrical potential. In the disclosed Lynam et al. structure, three substrates are required for two cells, and within the cells at least one liquid, gel or semi-solid electrochromic is used.
U.S. Pat. No. 4,135,790 to Takahashi et al. discloses a composite electrochromic cell that uses one colorant per pair of transparent electrodes.